Better biogas returns

Protecting power generating engines from harmful hydrogen sulphide (H2S) levels is an essential process requirement when producing renewable energy from sludge digesters - but as Yorkshire Water has found, only a complete picture of what gases are present and at what levels will avoid downtime and maximise revenue.

Yorkshire Water has set up a team to focus specifically on augmenting energy production - a remit that includes optimising sludge-digester performance, an often complex process. This work has led to the integration of more than 20 fixed-position, gas monitoring systems into Yorkshire Water’s main renewable energy control system.

The systems - from Coventry-based Gas Data - automatically monitor all sources of gas, with each digester monitored independently and the common gas mix that goes to the engines also recorded.

False economy
“It is false economy for end-users to only monitor the common gas going to the engines,” explains Chris Dakin, managing director of Gas Data. “This will allow you to tune the engines for the gas you can see and ensure that hydrogen-sulphide levels are under control, but gives no information whatsoever about performance and what gases are coming from which digester.”

But operators need to know how each digester is performing, as without appropriate knowledge from the analysers they can go round in circles, added Dakin: “An operator who doesn’t know exactly what gas constituents are present, can only say: ’Well, I don’t think there’s anything wrong with the digester’. This biogas resource is too valuable to be left to chance.”

Yorkshire Water first introduced the gas monitoring system four years ago at its sewage treatment works at Aldwarke, near Rotherham. This was identified as a prime site for using sludge-digester gas to generate on-site heating, with power also exported off site.

The water utility has also worked with Gas Data to develop a system that takes in all the analysed values from the instrumentation so that operations and controls can react accordingly.

Gas Data now independently logs all data directly from the analysers so that operators are fully aware of the gas status, and it has set up a website for off-site operators requiring remote access, alarms and maintenance threshold data. This information is sent by email or text as soon as required.

“A sludge digester is a bit like an intestine”, said Dakin. “The bacterial activity can be unpredictable, with the less ideal gases producing conditions that kill off the good, gas-producing bacteria.

“Temperatures will vary and undetected contaminants can cause gas production to fall - and then there’s the inevitability of hydrogen sulphide. It’s a complicated process, so all the more reason to monitor it properly.”

Spikes in the H2S level, which may cause serious odour problems, can be reduced with chemicals, but this is likely to reduce methane levels, affecting gas production and revenue. Protecting engines from HS has been paramount, with engine manufacturers requiring constant data on the gases being fed in.

Gas Data claims that its BOS biogas monitoring systems can provide cost savings of 39% on engine oil consumption and downtime - without causing any detrimental effect on engines or components. It can, it adds, almost double the lifetime of oil used in engines that generate renewable energy at sewage treatment works, biogas plants and landfills.

Since the first installation at Aldwarke, Gas Data’s LMSp fixed-position analysers have also been put into operation at Staveley, Hull, Bridlington, Naburn, Colburn and Lundwood - all of which see data gathered continually and then used to control the plant by direct connection with Gas Data.

The analysers monitor the digesters at least six times per day - as well as continuously monitoring the engines - and a full scan is carried out at least once per day.